The present invention relates to Vertical External Cavity Surface Emitting Laser devices (VeCSEL), and more specifically to the generation of two stabilized transverse modes with such devices.
The field of the invention is, but not limited to, semiconductor laser tera-Hertz (THz) sources and its applications.
THz waves cover the electromagnetic spectrum between the microwave and infrared. The domain of application is very wide with some promising applications, as for example in astronomy, radar systems, time-frequency metrology, bio-medical sensing and imaging, high-data-rate wireless communications or security. A large variety of solutions exist for THz emission, from optics to electronics, but existing solutions are often cumbersome or limited in terms of output power, coherence, tunability and/or modulation bandwidth: at the upper band of the THz spectrum, sources are usually pulsed and/or require low-temperature operation; and at the lower band of the THz spectrum, electronic-based sources usually operate easily at room temperature in a continuous-wave operation, and at a fixed frequency of emission. Moreover, frequency-multiplied sources offer enhanced tunability but do not provide simultaneously sufficient output power and high modulation bandwidth, compactness, or affordability.
Different techniques are implemented to generate THz waves, summarized into two categories:                The first one consists in two lasers setup, with photo-mixing experiments, for which the frequency difference between the two sources is widely tunable. In the other hand, as it uses two distinguish sources, it will be sensitive to the drift effects of both of them and thus the beating stability is weak.        The second one consists in a two modes laser: in this case the frequency difference is inherently more stable because the two modes will be sensitive to common effects and there will drift in a similar manner.        
Many well-known attempts to reach compact, stable, continuously-tunable, coherent and powerful dual-frequency lasers have been already proposed for terahertz generation, but none of them meets fully those requirements.
One of them is used to reduce mode coupling and consists in a separation of laser beams within a two-axis cavity. It shows some greats results in terms of tunability and coherence. However, tunability occurs by steps, output power is limited to few milliwatts, cavity is complex with intra-cavity movable elements, almost impossible to integrate on a single device, and coherence is degraded.
Another technique is the external-cavities diode laser using spectral filter(s) that can provide dual-frequency stability, but to the detriment of output power, cavity complexity, alignment sensitivity/robustness, beam quality, and coherence as the laser tends to operate two sets of longitudinal modes rather than two modes.
Vertical External Cavity Surface Emitting Laser devices (VeCSEL) are well known devices and are very promising solutions for dual-frequency lasers as they are inherently compact, wavelength flexible, widely tunable, powerful and highly coherent (spectrally, spatially and also in terms of polarization).
Dual-frequency laser operation based on the coexistence of two longitudinal modes for THz generation in a VeCSEL has already been described in M. Wichmann et al. “Systematic investigation of single and multi-mode operation in vertical-external-cavity surface-emitting lasers”, Proc. SPIE 89660N, but such design does not allow for simultaneous operation of both laser lines.
Other VeCSEL demonstration is described in Gaya Baili et al., “Experimental demonstration of a tunable dual-frequency semiconductor laser free of relaxation oscillations”, Optics Letters, Vol. 34, Issue 21, pp. 3421-3423 (2009). It deals with the use of a VeCSEL in the GHz range and based on dual polarization beam with intracavity beam splitting on the gain chip.
The aim of the present invention is to fulfil the previously mentioned problems and further to lead to some over advantages.
It is an object of the invention to provide stable and robust/controlled lasing effect of two transverse modes within a single cavity.
It is another object of the present invention to provide laser sources achieving a very high stability (i.e. high coherence or low phase noise) of the beating frequency.
It is another object of the present invention to provide laser sources achieving high-power operations with good coherence and good stability.
It is another object of the invention to provide laser sources achieving continuously or fine tunability over a broad frequency range.
It is another object of the invention to provide compact laser sources operating at room temperature in continuous wave.